1. Field of the Invention
The present invention relates generally to electrostatic discharge (ESD) protection in semiconductor devices, and in particular to providing ESD protection to input output devices by having current uniformity in the input output devices.
2. Description of the Related Art
ESD is the rapid, spontaneous transfer of electrostatic charge that occurs between bodies at different electrostatic potential and may be induced by direct contact or by an electrostatic field. ESD causes high current flow in electronic components. The high current flow can damage the electronic components. Also, the heat dissipated during ESD can be high and the corresponding high temperature can damage the electronic components. As a result, ESD protection has become a major focus for the electronic industry.
FIG. 1 is a simplified schematic diagram illustrating a transistor having a conventional ESD protection scheme. The transistor includes a drain 100, a gate 102, and a source 104. The transistor further includes poly-resistors 110 on the side of the drain 100 and the source 104. The ESD current flow is spread among the poly-resistors 110 so that the effect of the current will be divided among the numerous poly-resistors 110. In this manner, the intensity of the current flow to any particular poly-resistor 110 will be diminished, thereby preventing damage to the device. One of the shortcomings with this approach is that a silicide film is used for the resistors and the transistors for reducing the resistance of the drain, source, and gate regions. The narrow width of the silicide film leads to uneven distribution of its resistance profile and impacts the uniformity of the current flow. As the current will take the path of least resistance, the current will go through poly-resistors having the least resistance, caused by the uneven distribution of the silicide film, thus generating hot-spots where damage can occur. Also, the lower resistance caused by the silicide in the poly-resistor may enable the current due to ESD to flow to the gate region. Accordingly, there exists a high probability that the gate region will be damaged by the current from the ESD resulting in a damaged semiconductor device. Additionally, on the source side, the poly-resistors 110 cause a voltage drop in the source region. This voltage drop raises the source potential. In turn, the raised potential impedes the turn-on of the substrate, thereby making it hard to turn on the diode between the source and the substrate, which makes it more difficult to turn on the transistor.
In view of the foregoing, there is a need for semiconductor devices that can uniformly spread an ESD current, and further, there is a need for transistors that can be more easily turned on.